I love the source engine. So many different setups can run it. It's so flexible. I've played CSS and HL2 it on a PC with 256MB RAM and an FX5200. (not recommended though)
ok it has then been a while since I got back into these but I would chose the 6600 GT unless if you ever end up selling this card don't trust me all that mutch just because it might have a greater resale and you can then say that it costed more for this card. Like I said I just got back into these forums.